This invention relates to electronic signal conditioning circuitry and, more particularly, to circuitry adapted for level adjustment in aperiodic biological signals.
Biological and physiological phenomena are often examined by electronic apparatus. A common example of such apparatus is found in electrocardiology wherein electric wires are attached via terminals to selected sites on a person's chest. These wires couple faint electronic signals given off by pulsations of the heart muscle to signal processing and display equipment which produce graphical recordings of the waveforms of the electronic signals given off by the heart, such recordings being known as electrocardiograms. The various wires provide differing signal waveforms depending on the sites of their respective terminals. Further examples in the measurement of biological waveforms are found in encephalograms and in the study of speech. In each case, electrodes are attached to the subject to obtain the various signals for analysis of their respective waveforms.
As an example of the types of electronic equipment utilized in the measurement and analysis of biological signals, U.S. Pat. No. 3,602,706 of J. R. Levitt discloses a system useful for cardiograms and comprising a set of integrators arranged in feed-forward configuration and providing, in conjunction with a sine function computer, nonlinear transformations for visualization of heart function. An earlier embodiment of such a system, employing simpler circuitry, is shown in the U.S. Pat. 3,422,264 of J. R. Levitt.
Signal processors utilized in the analysis of biological signals, as well as signals obtained from other sources, may be fabricated of analog or digital circuitry. Today, it is usually the practice for such signal processors to employ digital circuits to take advantage of digital signal processing techniques. The biological signals sensed at the electrodes are in analog form. Accordingly, the digital signal processors employ analog to digital convertors for converting the format of the signals from digital to analog form. In a processor capable of high resolution of the signal waveforms, the A/D converters have many stages to provide the many bits of resolution as are required.
A problem arises in that the information contained in the sensed biological signals resides in only a fraction of the amplitude excursion. Typically, the signal is composed of a time-varying component superposed on a constant, or dc (direct current) level, component. In addition, the dc level may drift in amplitude at a rate which is slow compared to the rate of change in the time-varying component. Such drifting does not contain useful information and, accordingly, is to be deleted in the signal processing operation. The foregoing problem is also manifested in the need for analog to digital conversion which can extend over a wide range of values to accomodate the sum of both the varying component and the fixed component. This results in the need for far more extensive conversion equipment or, alternatively, a relaxation in the requirement for the number of bits of amplitude resolution to be used in the signal processing. In either case, the cost of the equipment is greater than desired for the amount of benefit received.